There have been many approaches in attempting to develop a reliable conveyor belt weighing system. Accurate motion weighing equipment is required for bulk handling of materials in many diverse industries, for example mining, ship loading, rail loading, grain, coal power, wool scouring, quarry, food industries, etc. Presently, however, no known belt weighing designs provide a sufficiently reliable live load performance on the basis of a static mass calibration. In the field of high accuracy belt weighing, particularly in the case of longer conveyer belt systems, there are two prevalent designs of belt weighing systems. These are: pivoted weigh frames (sometimes called “approach-retreat” systems); and, fully suspended or floating weigh frames.
Accurate weighing of materials whilst being transported on a conveyor belt poses some inherent problems. For example, a weighing scale suspension system must function to transmit forces on weighing system idlers (i.e. rollers) by transmitting only the force normal to the line of the conveyor belt without resolving any lateral forces or belt tension induced forces into the weigh frame output signal.
A particular problem that has been identified by the Applicants concerns the dynamic effects which occur as the material in transit on the conveyor rises and falls over each supporting idler set, these dynamic effects are caused either from relative movement of conveyed material along the conveyor belt or non-linear dynamic effects as the material rises and falls. Previously, the relative movement source of error has not been realised or has been deemed insignificant. The relative movement results in the actual particles of the bulk material not travelling along the conveyor belt at the speed of the conveyor belt itself. This relative movement error is often small, however, the relative movement error represents an error which has not previously been properly understood or appreciated, and which can cause belt weighing scales designed to achieve 0.25% accuracy in reality achieve only perhaps 0.5% or 1.0% percent accuracy.
One solution to this problem would be to measure the actual speed of the particles of the bulk material themselves by some means, and not to assume that the speed of the belt represents the speed of the conveyed material. However, this would necessitate development of new measurement systems.
The other source of error, thought to arise from non-linear dynamic effects, is not well understood. There are error effects which occur when material is weighed in motion, whether they are due to the non-linear dynamic effects or whether they are due to the material movement relative to the belt effect.
Another significant source of belt weighing error, that is known in the art, arises from the interactions between a weigh frame and a tensioned conveyor belt, via which the conveyed material must be weighed. It can be demonstrated that the precision of measurement of the weight per unit length of the conveyed material over the weigh frame is influenced by belt tension and idler (i.e. roller) misalignment. Ideally, if a conveyor belt was perfectly aligned and included a completely suspended weigh frame of ultimate rigidity, such that it was perfectly aligned at all times, then belt tension, a force in the belt which is perpendicular to the weight which is to be measured, would not have any influence on the weight measurement. However, ideal conditions do not normally exist and belt tension is a significant source of error in measurement of weight.
Referring to FIG. 1, there is illustrated a known conventional fully suspended weigh frame 110 forming part of a belt weighing system 100. Belt weighing system 100 includes idlers 120 spaced apart to support belt 130. Idlers 140 are part of fully suspended weigh frame 110. Conveyed material being transported along belt 130 imparts its weight via belt 130 and idlers 140 and can be measured by weigh frame 110. A fully suspended weigh frame has the property that if an idler mounted on the weigh frame were to jamb, then the resulting frictional force in line with the belt would have practically no effect on the output weight signal. Also, the normal idler rolling friction, nominally 2 to 3% of the load on the conveyor belt, is not reflected in the output weight signal from the weigh frame. In a fully suspended weigh frame, weight applied at any location on the weigh frame will produce the same output weight signal. Such designs are only sensitive to loads perpendicular to the conveyor belt.
Referring to FIG. 2, there is illustrated a known conventional dual pivoted “approach-retreat” type weigh frame forming part of a belt weighing system 200. Weigh frames 210a, 210b are pivoted at pivot points 215a and 215b, respectively. Idlers 220 support conveyor belt 230 and idlers 240 form part of the weigh frame. In pivoted weigh frame designs there is a component of idler friction in the output weight signal due to a turning moment about the pivot points. Weight applied to the weigh frame produces a variable output weight signal depending on the distance from a pivot point.
There is a need for a belt weighing system which addresses or at least ameliorates one or more problems inherent in the prior art.
The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.